Murder the phrases that you can't get exactly right. Delete the sentences that seem out of place. Annihilate entire paragraphs that don't say quite what you want to say.
Kill 'em all off. Have no mercy. Get rid of them and keep moving. They're mental clutter. Keeping them around just distracts you from your next great idea.
And if you're too wimpy to annihilate bad text, or if you're a hoarder of your personal writing, here's an alternative: Create a separate file on your computer where you can paste all the crappy phrases, sentences, paragraphs and book chapters that you just can't make work.
Keep them there in verbal stasis. You can always return to them in the future. But get that half-baked writing out of the piece you're working on right now. Get rid of it.